The primary goal of this study is to examine whether enhancing the exercise intervention either during the initiation phase of treatment (EX-ADOPT) or during the maintenance phase of treatment (EX-MAINTAIN) improves long-term weight loss in a behavioral weight loss program compared to a standard behavioral intervention (SBWP). Secondary aims of this study include examining changes in cardio-respiratory fitness, percent body fat and body fat distribution, physical activity, and dietary intake in SBWP compared to both EX-ADOPT and EX-MAINTAIN. We will also examine potential mediators of physical activity behaviors. The prevalence of obesity has been increasing over the past few decades, and it is now estimated that in excess of 50 percent of adults are overweight. Exercise has been shown to be an important component of behavioral weight loss programs, and exercise appears to be associated with improvements in long-term weight loss. However, based on data from our studies, it appears that there may be key periods where the intervention should focus on the exercise component. One period appears to be during the adoption phase of treatment, as exercise participation early in treatment is predictive of long-term weight loss. Alternatively, it does appear that there are critical periods when exercise begins to decrease and weight begins to increase. Using a combination of proven strategies for improving exercise, we will target these two approaches in our proposed enhanced interventions (EX-ADOPT and EX-MAINTAIN), and examine if either of these approaches is more effective than SBWP for long-term weight loss. It is hypothesized that both EX-ADOPT and EX-MAINTAIN will improve weight loss, physical activity, fitness, and body composition compared to the SBWP across an 18-month intervention. Assessment of weight, fitness, and body composition will occur at 0,6,12,and 18 months. To better understand the impact of potential mediators on exercise adoption and participation in this study, we propose to assess these factors and physical activity at 0,3,6,9,12,15, and 18 months. It is believed that this proposed study will lead to improvements in exercise participation in overweight women, which will result in improvements in the long-term treatment of obesity.